A series of tweets by U.S. President Donald Trump about the investigation into contacts between his 2016 campaign and Russia prompted concerns on Sunday among both Democratic and Republican lawmakers, with Republican Senator Lindsey Graham saying Trump could be wading into “peril” by commenting on the probe.

“I would just say this with the president: There’s an ongoing criminal investigation,” Graham said on the CBS program “Face the Nation.”

On Sunday morning, Trump wrote on Twitter that he never asked former FBI Director James Comey to stop investigating Michael Flynn, the president’s former national security adviser – a statement at odds with an account Comey himself has given.

That tweet followed one on Saturday in which Trump said: “I had to fire General Flynn because he lied to the Vice President (Mike Pence) and the FBI.”